Should I tell my sister-in-law about her husband's illegitimate son?

My brother is the only son in our family, so my parents tried their best to persuade him to return to Vietnam. He didn’t say anything about his relationship or the fact that he fathered a child.

Initially, he always tried to go back but our family made it difficult for him. He gradually became used to his life in Vietnam and started to build his career here.

A few years later, he met a girl he wanted to have a family with. My brother confessed to our mother that he had a child, and she advised him to be upfront about this with his girlfriend so she could make an informed decision about getting married to him.

However, my brother flatly refused. Our mother didn’t know anything beyond him having a son outside of marriage, so decided not to interfere and said that he had better not regret his decision later. My brother was 32.

His girlfriend has been my sister-in-law for 19 years now, and our entire family adores her. Ten years after my brother’s marriage, my mother told us sisters about the existence of our nephew, but we still had no idea about who he was, how he’s been, or the identity of the mother.

My brother never told our mother anything about this and obviously never said anything to us. We were worried about his child but had no way of contacting him. We also didn’t want to interfere in my brother’s family matters.

Two weeks ago, my brother held a family meeting, announcing that his child had gotten married and wanted to come to Vietnam to visit his family in two weeks. This means our nephew and his wife are arriving in Vietnam this week. My brother requested that we not say a word about this to our sister-in-law and he would tell her himself after their oldest son has completed his university entrance exams.

As his aunts, we want to welcome our nephew and his wife with open arms, but as our sister-in-law’s family members and women ourselves, we feel terrible for her.

If we were in her position, we would understand how betrayed and angry she must feel, being lied to by her entire extended family. My brother has been dishonest with his wife, and we don’t know what to do.

We won’t have to hide anything from our sister-in-law when we welcome our nephew and his wife, but there is a chance that the peace in her family will be disturbed, which could prevent their eldest son from doing well with his exams, causing my brother and his wife to resent us.

Other men in our family find nothing wrong with this entire situation, but for us sisters, imagining being in the same circumstances where we are kept in the dark by ones we trust has left us deeply uncomfortable about the whole ordeal.

Should I tell my sister-in-law this family secret, or stay silent?